This invention is related to a process for the production of purified 2,6-naphthalene dicarboxylic acid (hereafter referred to as 2,6-NDA) from a crude 2,6-NDA disproportionation product. More particularly, this invention is related to a novel method of separating and recycling byproducts in a process for producing 2,6-NDA from a disproportionation product that utilizes reverse osmosis and is industrially advantageous.
Aromatic carboxylic acids are highly useful organic compounds. They are useful as intermediates for the preparation of other organic compounds, and as monomers for the preparation of polymeric materials. In particular, the naphthalene carboxylic acids are used for preparing photographic chemicals and dyestuffs. Naphthalene dicarboxylic acids can also be used to prepare a variety of polyester and polyamide compositions. 2,6-NDA is a particularly useful aromatic carboxylic acid which can be reacted with ethylene glycol to prepare poly(ethylene-2,6-naphthalate). Polyesters prepared from 2,6-NDA have excellent heat resistance, gas barrier, and mechanical properties. Therefore, much research in the art has focused on methods of preparing 2,6-NDA. The production of 2,6-NDA from disproportionation product is described, for example, in U.S. Pat. Nos. 2,823,231 and 2,849,482.
Production of high purity 2,6-NDA from disproportionation product requires many process steps to separate impurities from the dipotassium salt of 2,6-NDA hereafter referred to as 2,6-K2NDA, which is the 2,6-NDA precursor. The impurities include naphthalene, zinc oxide, and several naphthalene mono- and dicarboxylic acid salts. This complexity results in numerous byproduct streams that must be recycled to make the process less costly.
There have been different approaches to the separation of the dialkali metal salt products of disproportionation reactions and conversion of them into 2,6-NDA.
In U.S. Pat. No. 2,823,231, the method used to separate the dialkali metal salts of 2,6-naphthalene dicarboxylic acid comprises dissolving the disproportionation conversion product mixture in water, filtering off insoluble impurities from the resulting solution, acidifying the filtrate with mineral or organic acid, such as sulfuric or hydrochloric acid, and separating the precipitated naphthalene-2,6-dicarboxylic acid from the acid solution. In U.S. Pat. No. 2,823,231 the dialkali metal salt of naphthalene 2,6-dicarboxylic acid formed is converted into free naphthalene 2,6-dicarboxylic acid by acidification of said dialkali metal salt with a strong mineral acid.
U.S. Pat. No. 2,849,482 teaches acidifying an aqueous solution of the crude reaction product of the disproportionation or converting the crude alkali metal salt directly into the dichloride or into esters of naphthalene-2,6-naphthalene dicarboxylic acid in accordance with known methods.
In U.S. Pat. No. 3,631,096, to Phillips, salts formed by the reaction can be transformed into the corresponding free acids by acidifying the solution with organic or inorganic acids or by introducing carbon dioxide into the solution at atmospheric or elevated pressure, and then separating the free acids from the acidified solution. The individual reaction products may be separated from each other and isolated in pure form by methods that are based upon their different solubilities or volatilities and may thereafter, if desired, be transformed into their derivatives. The salt mixture produced by the reaction may also be transformed directly into derivatives of the acids, for example, into their esters or halides, and these derivatives may be purified, if desired, by fractional distillation.
U.S. Pat. No. 3,671,578, to Teijin, discloses that the monoalkali salt of 2,6-naphthalene dicarboxylic acid is easily disproportionated when heated in water or water-containing organic solvent, to form free dicarboxylic acid and by-product dialkali salt, and the former acid is precipitated.
In U.S. Pat. No. 3,952,052, to Phillips, there is disclosed a process for separating a disproportionation reaction product by forming a slurry comprising alkali metal salts of aromatic polycarboxylic acid and dispersant and a gaseous effluent, and then lowering the pressure, flashing the dispersant, and recovering said alkali metal salts of said polycarboxylic acids as solids from said separation zone.
U.S. Pat. No. 3,888,921, to Teijin Ltd., discloses a method for purifying a dialkali salt of crude 2,6-naphthalene dicarboxylic acid comprising precipitating 40 to 97 mol percent of the dialkali 2,6-naphthalene dicarboxylate dissolved in an aqueous solution substantially as monoalkali salt of 2,6-naphthalenedicarboxylic acid while maintaining the pH of said aqueous solution at a value not lower than 6.3, and separating the precipitate, and converting the separated precipitate to 2,6-naphthalene dicarboxylic acid.
Canadian Patent 864587 discloses a process for the preparation of 2,6-NDA which comprises heating a monoalkali salt of 2,6-NDA in water or water-containing organic solvent causing disproportionation thereof into 2,6-NDA and a dialkali salt and separating the 2,6-NDA by a method that includes dissolving a rearrangement reaction product containing dialkali salt of 2,6-naphthalene dicarboxylic acid in warm water, filtering off the insoluble matter therefrom, concentrating the remaining solution, whereby the filtrate is concentrated to such a degree that the precipitation yield of the dialkali salt precipitated when the concentrated liquid is cooled to room temperature reaches at least 70% and the purity of said precipitate exceeds 99%, passing gaseous carbon dioxide through the aqueous solution of the precipitate recovered from the concentrated liquid, and recovering the resulting precipitate, and the mother liquour containing the side product dialkali salt of 2,6-naphthalene dicarboxylic acid is recycled into the carbon dioxide reaction step.
U.S. Pat. No. 5,175,354 discloses a reaction step wherein 2,6-naphthalene dicarboxylic acid potassium salts are allowed to react with benzene-carboxylic acids in the presence of water to yield 2,6-NDA and benzene-carboxylic acid potassium salts and a separation step wherein the crystallized 2,6-NDA is separated from the benzene-carboxylic acid potassium salts dissolved in the aqueous solution and provides 2,6-NDA.
None of these references suggest the idea of incorporating reverse osmosis membranes into a process for purifying 2,6-NDA.
There is a need in the art for alternative methods of separating the desired product and efficiently recycling byproducts. The purification process of the present invention provides an efficient way of separating and recycling byproducts which is advantageous.
In accordance with the foregoing the present invention comprises a process for purifying 2,6-naphthalene dicarboxylic acid produced by disproportionation and more efficiently recycling byproduct dipotassium salts which comprises:
a) Dissolving the disproportionation product of potassium naphthoate comprising the dipotassium salt of 2,6-NDA (K2NDA) in water, removing any residual disproportionation reaction medium, centrifuging the solution to remove the disproportionation catalyst, and removing acid salts other than 2,6-NDA by crystallization and/or carbon adsorption;
b) Contacting said aqueous solution of 2,6-K2NDA with carbon dioxide to form as a precipitate the monopotassium salt of 2,6-NDA (KHNDA) and an aqueous solution containing 2,3-KHNDA, K2NDA, and potassium bicarbonate;
c) Separating said monopotassium salt as a solid from said stream containing 2,3-KHNDA, K2NDA and potassium bicarbonate;
d) Disproportionating said monopotassium salt (KHNDA) to form solid 2,6-NDA and an aqueous solution containing K2NDA and potassium bicarbonate;
e) Separating said 2,6-NDA;
f) Concentrating said aqueous solution containing K2NDA and potassium bicarbonate from step (d) by reverse osmosis; and
e) Recycling concentrated K2NDA to step (b) and pure water to step (d).